docs: update calendar to match the 18.2 plan with the one announced

docs: update calendar to match the 18.2 plan with the one announced

Additionally, I've extended the 18.1 cycle by one more release,
tentatively assigned to Dylan, due to the ~2 weeks delay for 18.2.

Cc: Dylan Baker <dylan.c.baker@intel.com>
Cc: Juan A. Suarez <jasuarez@igalia.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>